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A4 vs Letter, and why the A sizes fit together so neatly

Most of the world prints on A4. The United States and Canada print on Letter. The two are close enough to be confused and different enough to cut off your footer.

Explainer · 3 min read · Updated 22 October 2026

A1A2A3A4A5A6A0 = 1 m². Each size is the one above, cut in half.1 : √2every A size

A4 measures 210 × 297 mm. US Letter measures 8.5 × 11 inches, which is 215.9 × 279.4 mm. So Letter is about 6 mm wider and A4 about 18 mm taller. Put one on top of the other and neither fully covers the other.

A4210 × 297 mmLetter216 × 279 mmLetter is 6 mm wider,A4 is 18 mm taller.
A4 (blue) and US Letter (dashed) at the same scale.

Where each one is used

A4 is standard almost everywhere: Europe, Africa, Asia, Oceania and most of South America. Letter, along with Legal (8.5 × 14 inches) and Tabloid (11 × 17), is standard in the United States and Canada, and common in Mexico, the Philippines and a few other countries.

The clever part: the A series

The A sizes come from the ISO 216 standard, which goes back to a German standard (DIN 476) from 1922. The idea is simple:

  • A0 has an area of exactly one square metre.
  • Every sheet has sides in the ratio 1 : √2 (about 1 : 1.414).
  • Cut a sheet in half across its long side and you get the next size, with the same proportions.

So two A4 pages side by side make an A3, and an A4 folded in half is A5. That's why enlarging A4 to A3 on a photocopier (141 percent) or shrinking A3 to A4 (71 percent) works without margins changing shape. Letter has no such relationship: two Letter pages side by side make 11 × 17 inches, which has different proportions again.

The B series sits between A sizes (B4 is between A3 and A4), and the C series is for envelopes: an A4 sheet fits flat into a C4 envelope, folded once into C5, twice into C6.

Printing A4 on Letter, and the other way round

Printing an A4 PDF on Letter paper at actual size cuts about 18 mm off the height, usually the footer. Printing Letter on A4 trims a few millimetres from the sides. Two fixes:

  • In the print dialog, choose Fit to page or Shrink to printable area. The page is scaled down by a few percent and nothing is lost.
  • Convert the PDF itself with Change page size, which fits every page onto the target size with the margins you choose. Do this before sending a document across the Atlantic.

In points

PDF measures pages in points, 72 to the inch. A4 is 595.28 × 841.89 points, Letter is 612 × 792. If you ever open a PDF's properties and see those numbers, now you know which paper it was made for. Our paper size calculator lists every common size in millimetres, inches, points and pixels.

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